The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized Millimeter Wave (mmW) frequency spectrum between 3G and 300G Hz for the next generation broadband cellular communication networks. The available spectrum of mmW band is two hundred times greater than the conventional cellular system. The mmW wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmW spectrum has wavelengths ranging from 1 mm to 100 mm. The very small wavelengths of the mmW spectrum enable large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions.
With recent advances in mmW semiconductor circuitry, mmW wireless system has become a promising solution for real implementation. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges for the mmW network. In general, a cellular network system is designed to achieve the following goals: 1) Serve many users with widely dynamical operation conditions simultaneously; 2) Robust to the dynamics in channel variation, traffic loading and different QoS requirement; and 3) Efficient utilization of resource such as bandwidth and power. Beamforming adds to the difficulty in achieving these goals. A robust control-signaling scheme is thus required to facilitate the beamforming operation in a challenging environment.
In cellular networks, pilot signals are needed for device identification and time-frequency synchronization. Primary synchronization signal is a unique signal with smaller search space, which can be used for first stage synchronization to achieve coarse frame boundary and frequency synchronization. Secondary synchronization signal is a unique signal with larger search space, which can be used for second stage synchronization to identify device and achieve fine (symbol level) timing and frequency synchronization. Reference signal is used for channel estimation and demodulation of data symbols. The three types of pilot signals for time-frequency synchronization and channel estimation introduce too much overhead. Furthermore, spatial synchronization is not considered in existing solutions (e.g., LTE). Future systems operate in much higher carrier frequency band that requires beamforming with very narrow beam width. As a result, synchronization signals need to align with TX and RX beams under spatial synchronization.
A beamforming system synchronization architecture is sought to allow the receiving devices to synchronize to the transmitting devices in time, frequency, and spatial domains in the most challenging situation.